NiceHash said in a statement it was "investigating the nature of the incident," and that it would close down its site for a day.

"We are working to verify the precise number of BTC (bitcoin) taken," the company said. "Clearly, this is a matter of deep concern and we are working hard to rectify the matter in the coming days.In addition to undertaking our own investigation, the incident has been reported to the relevant authorities and law enforcement and we are co-operating with them as a matter of urgency."

While NiceHash was unable to specify how much bitcoin had been stolen, users have pointed to a bitcoin wallet which holds 4,736.42 bitcoins — equivalent to $68 million.

Andrej P. Škraba, head of marketing at the firm, confirmed to news agency Reuters that around 4,700 bitcoins — worth roughly $68 million at current prices — were lost in the hack.

CNBC reached out to NiceHash Chief Executive Marko Kobal for comment but he was not available at the time of publication.

This wouldn't be the first time people have lost millions in cryptocurrency. In November, millions of dollars worth of Ethereum were "accidentally" frozen on the cryptocurrency wallet provider Parity after a user "suicided" the wallet, deleting its code and freezing all ether tokens contained within.

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